Tea is best kept fresh by putting in a dark and dry, sealed in a closed container until using (and then kept as close to sealed as possible). This should keep black tea fresh up to one year. We do not recommend putting in the freezer, in that it will add moisture to the tea leaves, greatly reducing the flavor.

By the way, "The Age of Tea" -- that's a great title... mind if I borrow it for my new book?

This isn't something I'd pick out, but since it was mentioned, it bugs me, too. There's no reason to abbreviate "before" as "b4" on a web board where there is no character limit to your messages. Maybe it makes sense in text messages on cell phones or something, but not in general. It saves very little time and can make things harder for others to read. I know that it takes me longer to interpret heavy use of chat-speak than properly written English. I don't think anyone's expecting perfect English 100% of the time, but you should be able to write a proper, coherent message that will be understood by everyone without needing chat-speak stuff.

Anyway, as for the original question, I believe that if you store tea properly, it can last a pretty long time. Black tea, being the most oxidized, will probably suffer the least from exposure to air. You'll still want to avoid it as much as possible and keep the tea in a tightly sealed container in a cool, dark place. The tea should lose some of it's flavor over time, but exactly how long it will take for you to notice the change, I don't know. Proper storage should go a long way to keeping the tea as fresh as possible for as long as possible.

It depends on what you've put into the tea and whether you've covered it or not.

If you've put milk or cream into the tea, I wouldn't let it go for more than a few hours (unless you've put it in the fridge). Same goes for tea with lemon or any other perishable add-in.

If you've put sugar in it but haven't covered it, it'll attract bugs, so I wouldn't let that go for more than a few hours either.

If you've put nothing in the tea, it won't go bad. However, if you've left it uncovered, it will collect dust or whatever's in the air. If you want to make tea ahead of time, I'd suggest brewing it, fixing it however you like, letting it cool to room temperature (covered), and then pouring it into a container and storing it in the fridge. This is perfect for iced tea or creamer/milk-free tea that you'd like to reheat later. I don't like the taste of microwaved tea - if you reheat it, I'd suggest using the stove (and covering it with a lid so that you don't lose liquid to steam). This also works well if you're storing tea with creamer/milk that you plan to serve iced.

If you're storing tea with creamer/milk that you plan to reheat, you may find that it's too much work for little reward - tea tends to taste best when freshly brewed; I've always felt that reheating it does something to the taste.

If you do plan on storing it in the fridge, make sure you use an airtight container as tea will easily pick up the smell or flavor of anything else you have in the fridge. (Imagine a nice cup of sencha flavored with...last night's pork roast? Not unless it's a crazy TeaChef experiment!)

Brewed tea is probably fine left out for about a day. However, if not refrigerated, brewed tea, even with no sugar or other added ingredients, can support mold growth. Therefore, I would refrigerate the tea if you're not planning on drinking it within a day's time.